Study of Simultaneous Hydrogen Gas Stream Purification and CO2 Storage in Coal Samples using an Experimentally Simulated Huff ’n’ Puff Scheme Combined with the Rate-Transient Analysis, Porosity, and Permeability Core Analysis Method

Summary Hydrogen (H2) production and storage are key strategies that enable the transition to low-carbon energy on a global scale. Steam methane reforming (SMR) is a common process for generating H2, but SMR produces carbon dioxide (CO2) as a byproduct, which needs to be separated from the gas mixture and sequestered to reduce the carbon footprint. One solution is to simultaneously purify the H2 component and permanently sequester CO2 by injecting mixtures of H2 and CO2 into depleted subsurface coal through an injection well and producing the gas mixtures through an offset production well. This process was recently simulated and verified experimentally using a coreflooding approach. In the current work, an experimentally simulated huff ’n’ puff (HNP) scheme is applied to evaluate the extent of H2 purification/CO2 storage that can be achieved. The rate-transient analysis, porosity, and permeability (RTAPK) method is also used to estimate permeability after each HNP cycle. In the current study, synthetic SMR gases were injected into a coal sample from the Mannville Formation of western Canada, allowed to soak, and produced from the coal sample. Two cycles of injection/equilibration/production of the synthetic SMR gas [volume percentages: 75% H2/15% CO2/10% methane (CH4)] were performed using RTAPK applied to the coal sample at reservoir temperature; a third cycle was implemented using the purified SMR gas after Cycle 2. For each cycle, the initial injection pressure was between 870 psi and 880 psi, followed by a soaking period of 30–150 hours. After the injection stage of Cycle 1, the gas composition stabilized to ~95% H2/2% CO2/3% CH4, demonstrating a simultaneous increase in H2 component free-gas concentration and decrease in CO2 and CH4 component concentrations. A similar result was obtained for Cycle 2. At the start of the production stage for Cycle 1, the produced gas was close to the equilibrated value, but at the end of this period, the produced gas composition was ~86% H2/6% CO2/8% CH4, indicating H2 concentrations were diluted due to the desorption of some CH4 and CO2 during production. A similar result was again obtained for Cycle 2 but with a somewhat different (suppressed in H2 composition) late-stage produced gas composition of ~81% H2/7% CO2/12% CH4. The late production-stage Cycle 2 gas stream was then reinjected to determine if H2 could be further purified (Cycle 3), but equilibrium compositions obtained after reinjection, and late-stage produced gas compositions, suggest that further purification was not possible. For Cycles 1–3, the amount of injected H2 that was recovered at the end of the production stage was approximately 20%, 38%, and 46%, respectively; the amount of injected CO2 stored was approximately 96%, 92%, and 79%, respectively. The permeability of the coal sample to H2, measured before SMR gas injection, was estimated to be ~0.03 md. However, after the SMR gas injection, the permeability of coal sample decreased to ~0.01 md. This proof-of-concept study using the RTAPK method demonstrates that simultaneous H2 purification and CO2 storage can be achieved through injection of H2/CO2/CH4-containing SMR gases into coal samples and subsequent production of the gas mixture.